Fibre optic ferrule and method of forming such ferrule

Abstract: A fibre optic ferrule (2) is provided with a fibre core (4) mounted within the axial centerline of the ferrule, and the end face of the fibre ferrule has a chamfered lead in edge (6). The ferrule (2) also has an end face (8) which is offset from the axial centerline by an angle (β). The chamfered lead-in edge (6) has edges (6a and 6b) proportionately angled such that the distance from the vertex of the polished end face to diametrically opposed edges of the convex surface are equal. Two such ferrules are in physical contact at the exact centerline of the fibre cores when the ferrules are disposed in an alignment sleeve (4).

Description: The subject of the invention relates to an optical fibre connector, and more particularly to the ferrule which holds the optical fibre. Optical fibre connectors are typically comprised of an optical fibre coaxially positioned in a ferrule, which is then retained in an outer housing or sleeve. Generally, a latching mechanism retains two fibers or a fibre and an electronic component in electrical engagement. Typical examples include a bayonet style ring, or a plastic outer housing having a resilient latching system. The composition of a ferrule is typically chosen depending on the application for which the fibre connectors are used, and more particularly by the optical quality of the system. For example, for a system requiring high return loss, the ferrule is typically comprised of ceramic or metal, but in other systems for example in automotive sensors where a low return loss is adequate, the ferrules, can be comprised of a plastic molded ferrule. In any event, the optical quality of the system is a function of the axial alignment of the fibre ends as well as how well the fibre ends butt together. With reference to Figure 1, a well known prior art system is shown schematically where two ferrules having a chamfered leading edge is provided and where the end face of the ferrule is flat, that is perpendicular to the axial direction. The fibres would be located in an aperture along the centerline of the ferrules such that when the two ferrules butt together, the two fibre end faces are also butted one against the other. In a system such as that shown in Figure 1, where the ferrules are comprised of a ceramic material, it is common to place the fibre within the ferrule, adhesively fix the fibre within the ferrule and grind the end face to provide a smooth polished surface. This provides the best butting engagement between the end faces as well as provides a highly polished fibre face to prevent light loss. With reference now to Figure 2, a later prior art system is shown where two ferrules are shown having a radiused end face, again where a fibre would be inserted within an aperture of the ferrule, wherein the convex surface at the end face provided precision abutment of the two ferrules along the centerline of the ferrules. This type of system prevents the two ferrules from butting together at any other location than at the axial centerline where the fibres are. With respect now to Figure 3, it is also known to offset the end face of the ferrule by an angle α where this angle is usually between 7-15°. The reasoning behind such an angled offset is to improve the return loss in the fibre system. While the offset has improved the return loss, a further problem is caused when the ferrule also requires a chamfered outer edge. As the surface is polished, the vertex of the convex surface CS is transferred to a position V as best shown in Figure 3 while the center of the fibre end face remains along the centerline of the ferrule at F. Such ferrules and fibres are polished by pressing the ferrule and fibre together against a grinding medium such that the ferrule and fibre are held at an angle relative to the grinding medium, thereby causing the offset angle. The problem results in that, when grinding, the high point of the convex surface is located at the vertex. This is due to the fact that the polishing medium "finds" its own center, and that center is the geometric center of the convex surface. Theoretically, if two ferrules could be polished identically, the two would butt together at the centerline of the ferrules. However, in practice, the difficulty occurs in that no two ferrules can be polished identically the same, such that in one case a ferrule may be polished as shown in Figure 3, while the next ferrule is polished as shown in Figure 4. It should be noted that the ferrule of Figure 3 is polished further down than that of Figure 4. The more material that is taken off the end face of the ferrule [whilst the curvature (in particular the radius of curvature) is kept constant], the further the vertex of the convex surface formed is offset, such that D1 in Figure 3 is greater than D2 in Figure 4. While these distances are small, when these two ferrules are butted one against the other, as shown in Figure 5, the ferrules do not butt together at the centerline of the fibers, but rather at a position offset from the centerline of the fibers as shown in the exploded view of Figure 5 causing separation of the fiber end faces by a distance "x". U.S. Patent 5,140,660 to Seiko Giken addresses this common problem, and in an attempt to solve this problem, configures a ferrule with a front cylindrical tip extending forward from the remainder of the body portion of the ferrule. The chamfered section is located on the main body portion and the only portion which is ground to a convex surface at an offset angle is the very tip portion. While this system overcomes the problem of the ferrules being mismatched as shown in Figure 5, the portion of the fibre extending forward in the tip of the ferrule is unsupported by the associated alignment sleeve, and could itself lead to fiber misalignment. An object of the invention then is to provide an optical fiber system, and more particularly an optical fiber ferrule having a convex surface which is angularly offset from the centerline of the ferrule, but where the vertex of the convex surface is centrally located with the fiber end face. The objects of the invention have been accomplished by providing a fiber optic ferrule for receiving an optical fiber therein, the ferrule comprising a cylindrical body terminated by a chamfered lead-in edge portion provided with a convex front face which is angularly offset by an acute angle β with respect to a plane normal to the ferrule longitudinal axis, the ferrule being intended to receive the optical fiber along its longitudinal axis, the ferrule being characterised in that the chamfered lead-in edge portion is asymmetrically shaped, thereby providing different lead-in angles at diametrically opposed longitudinal sides thereof, and disposed such that its intersection with the cylindrical ferrule body is also angularly offset with respect to a plane normal to the ferrule longitudinal axis by substantially the same acute angle β as that of the convex front face, said asymmetrical lead-in edge portion as well as said angularly offset convex front face being furthermore mutually arranged in a manner such that the distance from the vertex of the convex front face to a given point on the outer perimeter of said convex front face is identical to that from the vertex to a point diametrically opposed on said outer perimeter, whilst said vertex is coincident with the point of intersection of the ferrule longitudinal axis with the convex front face. The invention also provides a method for forming such a ferrule. Embodiments of the invention are defined in the subordinate claims. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a prior art system showing a flat end face on the chamfered edge of the ferrule and no angular offset; Figure 2 is a prior art system showing two ferrules having convex end faces with no angular offset directly at the end of the ferrule cylindrical bodies; Figures 3 and 4 show prior art ferrules having a chamfered outer edge on the ferrule, a convex polished surface, where the polished surface is angularly offset relative to a centerline of the fiber ferule; Figure 5 is a view showing the misalignment between the ferrules 3 and 4; Figures 6-8 show alternative embodiments of the present invention having various radii of curvature of the end face; and Figure 9 shows two fibers according to the present invention in butting relation where the exploded view shows that the ferrules butt together at the centerline of the fiber cores. With respect first to Figure 6, a fiber ferrule is shown at 2 comprising a fiber 4 where the end of the ferrule has a chamfered lead-in surface 6 and a convex surface 8. In the embodiment shown in Figure 6, the convex end face 8 has a radius of curvature of 15mm where the convex surface is offset from the centerline of the fiber ferrule by an angle β. As mentioned above, this angle β typically ranges from 7-15°. In the embodiment shown in Figure 6, β has a value of 9°. As also shown in Figure 6, the ferrule chamfered surface 6 is asymmetrically chamfered, such that the chamfered surface has different lead-in angle components 6a and 6b; where for the geometry chosen in Figure 6, where β=9°, those components are 6a = 30°, and 6b = 26°. As shown in Figure 7, the asymmetrical chamfer surfaces 26a, 26b define a convex front surface such that, the distances x 1 and x 2 , from the vertex 30 to diametrically opposed points on the outer perimeter of the convex front surface are equal (wherein said outer perimeter is formed by the intersection of said convex front surface with the chamfered lead-in surface 26), while at the same time, maintaining the vertex 30 coincident with the fibre centerline at the convex front face. This remains constant regardless of how much ferrule material is ground from the end face, so long as the offset angle remains the same. Figures 7 and 8 show ferrules 22 and 32 respectively having different radii of curvature at the front convex surface, as well as the necessary geometry of the chamfered lead-in edge. As shown in Figure 9 two fibres terminated within ferrules similar to that of Figure 6 are shown terminated within an alignment sleeve 14. As shown in the exploded view, the two ferrules are physically contacting at the exact centerline of the fibres thereby ensuring maximum light transmission between the two fibres. In the preferred embodiment of the invention, the ferrules 2, 22 or 32 (Figures 6-8) are comprised of a ceramic material. This material could be extruded or moulded into a configuration comprising the cylindrical shape required for the ferrule. The chamfered lead-in edge 6 could be formed by grinding, or it could also be produced by other means, for example by initially moulding the lead-in edge into the ceramic ferrule. It should also be appreciated that the invention is useable with various radii of curvature of the convex end face, as well as with various offset angles β (Figure 6) of the end face. It is further anticipated that the invention is also useable with other ferrule materials such as metal or plastic. Advantageously then, the above mentioned invention provides for an optical fibre ferrule and a method of producing the same, where a fibre ferrule can include a chamfered lead-in edge, and where the end face of the ferrule can be offset relative to the axial centerline of the fiber ferrule, yet maintain the vertex of the ferrule end face coincident with the centerline of the ferrule and fibre. Also advantageously, this vertex is maintained at the centerline of the ferrule and fibre regardless of the degree of grinding or polishing of the end face. This provides that if the end face of the ferrule or fibre are damaged, the end face of the ferrule can be further polished, removing the damaged material, and yet maintain perfect light transmission qualities with the mating fibre and ferrule.

Claims: A fiber optic ferrule (2,22,32) for receiving an optical fiber (4) therein, the ferrule comprising a cylindrical body terminated by a chamfered lead-in edge portion (6) provided with a convex front face (8) which is angularly offset by an acute angle β with respect to a plane normal to the ferrule longitudinal axis, the ferrule being intended to receive the optical fiber along its longitudinal axis, the ferrule being characterised in that the chamfered lead-in edge portion (6) is asymmetrically shaped, thereby providing different lead-in angles (6a, 6b) at diametrically opposed longitudinal sides thereof, and disposed such that its intersection (7) with the cylindrical ferrule body is also angularly offset with respect to a plane normal to the ferrule longitudinal axis by substantially the same acute angle β as that of the convex front face, said asymmetrical lead-in edge portion (6) as well as said angularly offset convex front face (8) being furthermore mutually arranged in a manner such that the distance from the vertex (10, 30) of the convex front face to a given point on the outer perimeter of said convex front face is identical to that from the vertex to a point diametrically opposed on said outer perimeter, whilst said vertex (10, 30) is coincident with the point of intersection of the ferrule longitudinal axis with the convex front face. The fiber optic ferrule according to claim 1, characterised in that the convex front face is a partial spheroid. A fiber optic ferrule assembly comprising a ferrule according to claim 1 or 2, along the longitudinal axis of which an optical fiber is received, wherein the vertex of the convex front face substantially coincides with the point of intersection of the fiber axis with the convex front face. The fiber optic ferrule assembly according to claim 1, 2 or 3 characterised in that it is included in a fiber optic connector (14). A method of forming a fiber optic ferrule according to any preceding claim characterised by the steps of: asymmetrically chamfering a lead-in edge portion of a cylindrical ferrule body such that the lead-in angle at diametrically opposing longitudinal sides of the chamfered lead-in edge portion (6) thus formed has different angular values (6a, 6b) and the intersection of said chamfered portion with the ferrule body is angularly offset by an acute angle β with respect to a plane normal to the ferrule longitudinal axis, and subsequently polishing the end face (8) of the chamfered lead-in edge portion (6) thus formed such that it is made convex and also angularly offset with respect to a plane normal to the ferrule longitudinal axis by substantially the same acute angle β as that of the intersection of the chamfered portion with the ferrule body, wherein said asymmetrical lead-in edge portion (6) as well as said angularly offset convex front face (8) are furthermore mutually formed in a manner such that the distance from the vertex (10, 30) of the convex front face to a given point on the outer perimeter of said convex front face is identical to that from the vertex to a point diametrically opposed on said outer perimeter, whilst said vertex (10, 30) is coincident with the point of intersection of the ferule longitudinal axis with the convex front face.

Inventors: HULTERMANS ANTONIUS PETRUS, HULTERMANS, ANTONIUS PETRUS CORNELIUS MARIA
Applicants: WHITAKER CORP, THE WHITAKER CORPORATION
Application year: 1994
Main classification: G02B6
IPC codes: G02B6, B24B19